Magnetic flowmeter



F I P85 0 2. XR 15 8 05 342 Oct. 24, 1961 v. P. HEAD 3,005,342

MAGNETIC FLOWMETER Filed Oct. 21, 1958 SSheets-Shsst 1 FIG.

INVENTOR.

VICTOR R HEAD BY ATTORNEYS Oct. 24, 1961 v. P. HEAD MAGNETIC FLOWMETER 5Sheets-Sheet 2 Filed Oct. 21, 1958 FIG.

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3 1 m: I a .3 2 5 2 :3 A a 2 G u 2 n 6 w F .u. 2 2 H m 2 w m 2 2 M M 2 aK B 2 A D ||V l l 2 VICTOR P. HEAD Oct. 24, 1961 v. P. HEAD MAGNETICFLOWMETER 3 Sheets-Sheet 5 Filed Oct. 21, 1958 co ,5 INVENTOR. V l CT 0R P. H E A D ATTORNEYS Unite; Stat This invention relates to magneticflowmeters of the type in which a potential induced in a liquid flowingthrough a magnetic field is utilized as a measure of quantity of flow.

As is well known, the current or potential outputs produced In aflowmeter of the magnetic type are very small for ordinary flows to bemeasured and strong magnetic fields are required with consequentdifliculties in elimination of the disturbing effects of noise. It isthe general object of the present invention to provide flowmeters ofthis type by which reliable and accurate results may be secured.Hereafter described in detail is a flowmeter having various specialfeatures as follows:

High accuracy of measurement may be secured at low rates of linear flowof liquids of low conductivity; for example, accuracies of i1% at onefoot per second when the conductivity is as little as 20 micromhos percentimeter.

A high degree of insensitivity to lead lengths is secured makingpossible the location of recording apparatus quite remote from theprimary unit of the meter.

Quick responses to changes in flow rate are obtainable.

An electrical zero setting arrangement is built into the apparatus tomake possible the measurement of bidirectional flows.

Expansion is provided for any part of the meter scale.

A smoothing control is provided to minimize responses due to flowpulsations or effectively damp them out without destroying the overallinstrument response time. The arrangement used for accomplishing this isan electrical equivalent of a mechanical backlash arrangement. This isin contrast with the use of a dashpot or similar arrangement whichinherently greatly enlarges response time.

A built-in meter and associated adjustment is provided to secureinstrument phasing and balancing out of extraneous quadrature noise soas to allow a user of the flowmeter to assure himself that extraneouseffects are not rendering the readings inaccurate.

The flowmeter is of such type that there is ready interchangeability ofdetector or pickup units used in association with the same recordingunit. This feature lends itself to considerable economy in manufacturein that the recording unit may be standardized for utilization withprimary units involving very different flow ranges.

Improved means is provided for securing a uniform magnetic field in theregion of the pickup electrodes.

An advantageous electrode construction is provided to insure properoperation in the measurement of flow of such liquids as sewage which,with ordinary electrodes may produce coating thereof with insulating oilor grease.

Various aspects of the following disclosure are claimed in otherapplications filed of even date herewith. The general electricalcircuitry involved is the invention of Sholom Kass and is claimed in hisapplication, Serial Number 768,595. The adjusting means for securinginstrument phasing and balancing out of extraneous quadrature noise isof more general applicability than to magnetic flowmeters and is claimedin my application, Serial Number 768,701. The means for effectingminimizing or damping of response pulsations is also of more generalapplicability and is the invention of Sholom Kass and is claimed in hisapplication, Serial Number 768,762.

The present application is directed particularly to my inventions ofmeans for providing a uniform magnetic field and of improved electrodeconstruction.

The attainment of the, foregoing results and of the stated otherobjectives of the inventions will become more apparent from thefollowing description, read in conjunction with the accompanyingdrawings, in which:

FIGURE 1 is a plan view of the primary unit of a flowmeter embodying theforegoing inventions, the lower half of this figure showing an axialsection;

FIGURE 2 is a diagrammatic transverse section taken on the planeindicated at 2-2 in FIGURE 1;

FIGURE 3 is a diagrammatic longitudinal section taken along an axialplane at right angles to that involved in FIGURE 1;

FIGURE 4 is a wiring diagram showing the electrical aspects of theflowmeter; and

FIGURE 5 is an axial section through an electrode assembly particularlydesigned for the measurement of flow of sewage or other liquids whichcontain components which may contaminate electrode surfaces.

For presentation of a full background for the inventions claimed hereinthere will be described in detail a complete magnetic flowmeter whichembodies all of the inventions previously noted.

Referring first to FIGURES 1, 2 and 3, the meter comprises a stainlesssteel tube 2 provided with end flanges 4 by means of which externalconnections may be made to direct through the tube 2 the liquidundergoing measurement. The tube 2 is lined with an insulating materialsuch as neoprene which is continued about the faces of the flanges 8 toprovide electrical insulation and at the same time to prevent corrosion.At diametrically opposite points, the tube 2 is provided with openingsthrough which extend bosses of the lining 6, these being indicated at 10and provided with radial openings through which stainless steelelectrode pins 12 extend in fluid tight fashion with exposure of theirinner ends to the liquid flowing through the tube. Metal shielding caps16 provide arcuate channels in which are located leads 18 from theelectrode pins 12 which are brought together and extend exteriorly inthe form of twisted leads as indicated at 20.

It will be evident that the tube 2 may be of insulating material, suchas a plastic, in which case the electrodes may be carried in simplefashion by the tube walls, and the special insulating liner isunnecessary.

A magnetic field of uniform type is provided which extends at rightangles to the diameter forming the com.- mon axis of the electrode pins12 and to the tube axis, this field being provided by a pair of coils 22of identical shape. Each of these coils 22 comprises a coil of wirearranged as a pad to surround the opposite sides of the tube assembly,each coil being provided with a central opening indicated at 23 ofapproximately rectangular projected shape, with each coil provided witha semi-cylindrical open region 24 for embracing the flow tube assemblyand with a transverse generally rectangular opening 26 through whichlaminated core elements extend. Sections of the portions of the coilsextending parallel to the tube axis by planes normal to that axis areapproximately rectangular as shown in FIGURE 2. A rectangular type coresurrounds the coils and desirably is provided by sets of laminations ofmagnet iron as indicated at 28 and 30. The foregoing elements areenclosed within a shielding housing provided by steel casting halves 32and 34, there being associated with the assembly various mountingelements which need not be described in detail but which serve to holdall of the parts in fixed assembly to prevent vibration and change ofconfiguration of the coils. The leads from the electrode pins 12 arebrought out of the housing through a connector indicated at 36.

Dimensions of the coil and core assembly are chosen to provide a uniformmagnetic field. To secure this, the interior width W of the space housedby the coil desirably should not be less than 1.2 L, L being the lengthof the flux gap bounded by iron. The interior Width W bounded by thecore outside the coils is desirably not less than 1.6 D, D being thediameter of the tube bore if the bore is circular, or the effective oraverage diameter if the bore is non-circular, e.g. rectangular. The gapG between the coils is desirably between 0.1 D and 0.3 D with an optimumvalue of about 0.23 D in large size meters wherein W is close to 1.6 Dor 1.7 D. In small meters wherein W may be desirably in the range 3 D toD the gap G may be small and merely incidental to the existence of theseparate coils. The axial length A of the magnetic iron shell, theeffective axial length of the flux air gap, should be approximately 2 Dor greater. Using the conditions just stated with the generallyrectangular coils illustrated highly satisfactory uniformity of themagnetic field throughout the crosssection of the tube is securedconsistently with the use of a ferro-magnetic core having itsgap-providing surfaces closely approaching the tube and providing a maximum field strength for a given amount of coil copper.

In accordance with the invention the flowmeter comprises two units whichmay be conveniently referred to as primary and secondary units. Theprimary unit encompasses the elements which are located within theboundary 48 shown in FIGURE 4, this unit including the tube 2, theelectrodes 12, the coils 22 and the other physical elements alreadydescribed as well as certain electrical elements referred to later.Those elements shown in FIGURE 4 which are outside the boundary 48constitute the secondary unit. This devision of the flowmeter into twounits is convenient inasmuch as the secondary unit may be standard forflowmeters of a large range of flow capacity while the primary units maybe made different for the measurements of diiferent flow ranges. All ofthe primary units are arranged to match the standardized secondary unitto afford interchangeability.

Consideration may now be given primarily to FIG- URE 4. While notdetailed, it will be understood that careful shielding is provided inaccordance with good practice throughout the electrical system to avoidpickup of undesirable signal components. However, even with the mosteffective shielding there Would still occur spurious signals, and aswill become more apparent hereafter the circuitry adopted is such as toproduce balancing out of spurious signals so as to minimize theireffects on the flow readings and make possible the accuracies alreadyreferred to. The general circuitry involved is the invention of SholomKass and is claimed in his application, Serial Number 768,595.

Alternating current is supplied from the terminals 50 connected to theusual power supply, for example, 110 volts at 60 cycles. The diagramalso shows various direct current supply terminals and it will beunderstood that these are fed by conventional direct power suppliesenergized from the commercial alternating supply.

The magnetic field windings 22 are connected in parallel and to thesupply terminals 50, there being in series with the field windings theprimary winding 52 of a toroidal transformer 54 the secondary winding 56of which is connected to a network comprising the capacitor 58 connectedacross the secondary and the parallel resistance arrangement comprisingin series the fixed and adjustable resistors '60 and 62 and theadjustable resistor 64, to the terminals of the latter there beingconnected the leads 66 to the secondary unit. The transformer 54 and thenetwork provide the impedance match to the secondary unit. By the use ofthe network described, the output of the secondary of the toroidalcurrent transformer 54 is adjusted to be 180 out of phase (from thestandpoint of its feedback action later described) with the signalpotential which appears at the electrodes 12, it being noted that theprimary of this current transformer is directly in series with thewindings 22 and carries the current in these windings. While theelements of the network are interdependent, the adjustment of resistor62 primarily affords phase adjustment while that of resistor 64 aifordsamplitude adjustment. These provide corrections for eddy current shifts.The result of the adjustments is to provide a constant ratio between thepotential per unit velocity appearing at the electrodes and the currentwhich is provided a the conductors 66. The ultimate result is that theresponse of the secondary unit is full scale in terms of feet per secondof liquid flow velocity for any primary unit which may be associatedwith a secondary unit, the transformer 54 having a turn ratio consistentwith the securing of this result.

The leads from the electrodes 12 are connected individually through thesecondaries 68 and 70 of identical transformers 72 and 74 and throughthe capacitors 76 and 78 to the grids of triodes 80 and 82. Theprimaries of the transformers 72 and 74 are connected in parallelbetween ground at 84 and a line 86 in such fashion that signals fed backthrough the line 86 will null the signals from the electrodes, theconnections being such that opposition to the electrode potential isprovided by each transformer. The symmetrical arrangement here adoptedinvolves rejection of signals which may flow in the same directionthrough the symmetrical connections.

The feedback signal in connection 86 is derived from a network receivingits input from the lines 66. A potentiometer 88 connected between theselines has its adjustable contact 90 grounded. A second potentiometer 92is connected between these lines to provide a variable resistance. Athird potentiometer 94 connected between these lines has its adjustablecontact 96 connected through capacitor 98 to the connection 86. A fourthpotentiometer 100 connected between the lines 66 is arranged asillustrated with its variable contact 102 connected to one of the linesthrough a resistor 104 and through a variable resistor 106 and a fixedresistor 108 to the range adjustment network generally indicated at'110. This comprises a group of ganged switches 112, 114, 116, 118 and120 connected as illustrated between the resistor 108 and ground inconjunction with the equal resistors 122 and 124 having fixed values.The series arrangement of fixed resistor 128 and the adjustable resistor126, and the potentiometer 130, the adjustable contact 132 of which isconnected through resistor 134 to the resistor 108 and through resistor136 to the connection 86, provides an output to the connection 86. Apair of small capacitances are connected in parallel between theconnection 86 and ground. As will more fully appear, the potentiometercontact 102 is adjusted by a reversible motor 188. The functions of thevarious parts of the network just described are as follows:

The potentiometer 88 serves as an electrical centering control to setzero flow at any desired position on the recording chart of the meter.This makes it possible to indicate and measure bidirectional fiow Wherethat is required. The nature of this action will be evident uponconsidering the ground connections of contact 90 and, at 84, the groundconnection of the primaries of transformers 72 and 74.

The adjustable resistance at 92 is to set the input resistance of thebalancing network. This input resistance is desirably of low value,typically, for example, about 81 ohms, and by the use of the adjustmentunder discussion the input resistance may be set to such a value thatvarious secondary units may be made interchangeable.

The potentiometer 94 and its connection through capacitor 98 provides anull control allowing an operator to null out unwanted signals which arein quadrature with the error signal and aids in phasing the servoamplifier precisely, with greater accuracy than is attained by using anoscilloscope. The proper phase of quadrature signal is obtained by useof the capacitor 98 the reactance of which is many times that of thetotal net work. A phase shift obtained from this capacitor is verynearly 90 and the shift gives essentially a true quadrature signal.

Balancing is effected by the motor controlled movements of the contact102 of potentiometer 100 which is associated with the fixed resistor 104which compensates for the load on the potentiometer 100 caused by therange adjustment network, and with the adjustable resistance 106 whichcompensates for the loading of the range network by the input impedanceof the balanced transformers 72 and 74.

The balance signal from the last mentioned elements is fed andattenuated through the range adjusting network 110 so that full scalesensitivity is accurately known. The adjustable resistor at 126 servesfor trimming. The range potentiometer 130 is desirably of multiturn typeand constitutes in conjunction with resistors 122 and 124 a voltagedivider network. With resistors 122 and 124 equal (for example, havingvalues of 450 ohms each) and with the parallel arrangement of thepotentiometer 130 and the adjustable and fixed resistors in paralleltherewith providing an effective resistance of the same value (thepotentiometer 130 having, for example, a resistance of 500 ohms), theswitching arrangement is such as to locate the potentiometer in any oneof three alternate positions in a series arrangement including it andthe resistors 122 and 124. Thus, considering an arbitrary overall rangeof 0 to 30, the placement of the potentiometer resistance may be in arange 0 to 10, to 20 or 20 to 30, depending on the position of theswitches, so that full range adjustment of the potentiometer may occurthroughout any of these ranges.

The inductive reactance of the balancing transformers 72 and 74 causes aphase shift of the balancing signal which must be corrected, and thiscorrection is obtained through the use of capacitors 138 which may bechosen to suit particular units since the necessary correction variesfrom unit to unit. Through the use of standard capacitors, one beingrelatively large and the other being small to act as a trimmer, it isunnecessary to provide for this phase correction an adjustablecapacitor.

If it were assumed that there was an indicator of the potentialdifference between the grids of triodes 80 and 82, and if adjustment ofthe contact 102 of potentiometer 100 was made to provide a zeropotential difference at these grids, i.e., a null, it will be evidentthat the setting of the potentiometer contact would be a measure of theliquid flow. Automatic adjustment to secure a null is achieved as willnow be described.

The triodes 80 and 82 and their associated circuitry provide apreamplifier for the net output from the secondaries of transformers 72and 74 and the electrodes 12. In this connection it may be noted thatthese transformers may be desirably located in the primary unit assemblyto reduce the effect of cable capacitance as a shunt of signalsoriginating in high resistance liquid. In such case the capacitors 138should also be in the primary unit since they correct for the phaseshift due to the inductive reactance of the transformers. A cableconnection may thus either precede or follow these transformersecondaries for their connection to the remaining portions of thecircuitry. The preamplifier provides primarily an impedance matchingdevice and transformation from a balance-to-unbalance arrangement. Thetriodes are connected in push-pull arrangement to the primary windings140 of a transformer 142, the secondary 144 of which feeds through atransformer 150 amplified signals to the first stage triode 152 of themain amplifier. Desirably, special filtering is provided at 148 for thepositive supply provided to the triodes 80 and 82 from a positive supplyterminal 146 of the power supply. The main amplifier involves thetriodes 152, 154, 156 and .164 in generally conventional form with phaseshift adjustment by variation of contact 153 of potentiometer 151,providing a variable resistance associated with capacitor 155, and withgain control provided at potentiometer 158. In order to avoid hum it isdesirable to provide to the heaters of triodes 80, 82, 152 and 154suitable direct current which may be derived from the supply through asuitable rectifier and simple filter system, not shown. Rate feedbackcontrol is provided at the potentiometer in the cathode-to-ground returnof triode 156.

In order to provide suflicient motor driving power, a pair of triodes166 and 168 provide a power amplifier by arrangement in parallel. Theiroutput is provided through resistor 170 to the field winding 172 of themotor 188. The other phase winding of this motor indicated at 186 isprovided with reference current from the terminals 50 through thecapacitor 187. It will be understood that the motor is of a type whichreverses in accordance with the phase relationship of the currentsthrough its windings 172 and 186, remaining stationary when the currentin winding 172 is in quadrature with that properly produced therein bydesired signals picked up by electrodes 12. Shunted across the fieldwinding'172 is the series arrangement of an alternating currentvoltmeter 173 and a capacitor 175. The purpose of this arrangement willbe more fully described hereafter.

The primary 174 of a transformer 176 is connected between the signaloutput side of the resistor 170 and the parallel arrangement of resistor180 and capacitor 178, the right hand end of the transformer primary 174being connected through resistor 182 to a positive supply terminal whichmay be the same terminal as that to which the winding 172 is connected.The secondary of the transformer 176 provides a signal between groundand the adjustable contact of the rate adjustment potentiometer 160, theconnection being through resistor 184 and lead 162. This rate feedbackcontrol has its usual functions.

In a fiowmeter of this type hydraulic noise may cause rapid excursionsof a recording pen producing a broad line on the chart and this isundesirable. Heretofore, these excursions have been damped out by theuse of dashpots, but they, in turn, greatly slow down the response. Inthe present system provision is made electrically (by means forming thesubject matter of said Kass application, Serial Number 768,762) for anaction which corresponds, roughly, to the use of backlash in mechanicalgearing but with provision for proper adjustment. In brief, this isaccomplished by operating the final amplifier stage comprising thetriodes !166 and 168 under class C conditions thereby limiting theresponse to large signal excursions only, in excess of those which wouldbe due to noise. To secure this result, variable bias is applied to thegrids of these last stage triodes. A contact 192 of potentiometer 194 isdriven by the motor 188 to provide automatic adjustment. Thepotentiometer 194 is connected in series with a resistor 196 betweenground and a negative bias supply terminal 198 of the power supply. Alsobetween this terminal 198 and ground are the resistor 200 andpotentiometer 202, the adjustable contact 204 of which is connectedthrough resistor 206 to the grids of the final stage triodes 166 and168. The potentiometer contact 192 is joined to the junction of resistor200 and potentiometer 202. The network just described is providedbecause hydraulic noise is not precisely proportional to flow rate. Theresistance network comprising 194, 196 and 200 provides an outputvoltage across potentiometer 202 so that the smoothing control thusconstituted has the desired voltage characteristic. Potentiometer 202has a high resistance as compared with the other resistances in thissmoothing network and consequently does not alter the characteristic ofthe output voltage but is a manually adjustable amplitude control usedonly to limit the bias voltage as dictated by the hydraulic noise of thesystem. The ganging of the contact 192 with contact 102 determines therelative amount of smoothing voltage applied as a bias to the amplifierstage.

Because of the application of the negative bias varied in accordancewith desired operation, the last amplifier stage operates under class Cconditions so that signal excursions less than a predetermined amplitudedo not produce motor-driving output. The amount of the bias determinsthe minimum signals received from the triode stage 164 which willproduce motor drive. The amount of this minimum may be manually adjustedthrough the potentiometer contact 204, while the amount is alsoautomatically adjusted by the operation of the motor 188, the range ofthe dead region within which the drive will not be effected varying withthe flow rate as reflected by the position of the motor 188. In general,the range of this dead region is desirably greater for larger flow ratesthan for smaller flow rates. Further, it is desirable for the Width ofthe dead region to become essentially zero, as a result of operation ofcontact 192, when the flow is somewhat greater than zero, to assure alive zero, and the constants of the circuit are chosen accordingly.

While 188 has been generally referred to as a motor, it will beunderstood that this will generally be a conventional phase-sensitiverecorder motor driving through reduction gearing the potentiometercontacts 102 and 192 and either an indicator or a marking pencooperating with either a fixed or time driven chart scale indicated at190. In conventional fashion this may also (or solely) operate controlsrelated to the flow, e.g. to maintain the flow constant, to effect otheroperations in accordance with the flow, or the like.

The overall operation of the flowmeter may now be briefly outlined asfollows:

For a given rate of flow through the tube 2 there Will be produced anoutput voltage across the electrodes 12 the magnitude of which isproportional to the flow rate for a given magnetic field strengthprovided by the windings 22. Prior to balance corresponding signals areapplied to the amplifier system to provide to the motor winding 172 acurrent which will drive the motor 188 and with it the potentiometercontact 102 to provide a feedback signal to balance the electrode signalto provide a zero input to the amplifier. In case of a voltage change atterminals 50 affecting the strength of the magnetic field, acorresponding change in output from the transformer 54- occurs so as tobalance out eifectively such variations. Adjustments which have alreadybeen described take care of quadrature potentials which enter into thesystem.

The foregoing assumes error signals of suificient magnitude to drive themotor 188. The smoothing arrangement providing bias to the last stage ofthe amplifier prevents such movements when the error signals due tonoise fluctuations are insufiicient to provide output from the class Camplifier stage. Despite the fact that small fluctuations will notinvolve response of the motor, it should be noted that the class Coperation referred to does not involve any deterioration of response tosignals exceeding those which are to be effectively suppressed. Thusthere is no loss in rapidity of response to desired signals.

The use of the meter 173 for adjustment may now be described. (This isclaimed in my application, Serial Number 768,701 referred to above.)Potentiometer 96 serves to introduce a quadrature signal to the line 86.The introduction of a quadrature potential amplified and delivered tothe motor winding 172 (as well as to the voltmeter 173) should produceno rotation of the motor, i.e., no change in flow indication, if thecorrect phase relation between the amplifier input and the motor fieldcurrent exists. A test for proper amplifier phase adjustment may be madeby manually moving the potentiometer contact 96 in both directions sothat the voltmeter 173 reveals amplified quadrature signals. As a resultof such signal changes there should be no change in flow indication.

If such change occurs, it is necessary to adjust the phase of theamplifier output by change of the setting of contact 153 ofpotentiometer 151. It will be apparent that when the quadrature signal,which is always at with respect to the flow signal, produces nodeflection of the flow indicator, the phase relationship between theamplifier input and the servomotor field will be that for maximumsensitivity to the flow signal and for optimum rejection of any spuriousquadrature signal that may arise in the flow detector. When the phase ofthe amplifier has been properly adjusted at 153 so that movement ofpotentiometer contact 96 and correspondingly large variations inreadings on meter 173 are no longer accompanied by changes in flowindication, the contact 96 is then adjusted to give the minimumachievable reading on meter 173, the significance of which minimumreading would be that minimum quadrature signals were being introducedto the amplifier. Small phase drifts with time in the amplifier can thenproduce no significant error in flow indication in the absence ofsubstantial quadrature signals. Conversely, if, as would be unlikely, aspurious quadrature signal appeared over a long time period, no error offlow indication would result so long as the phase situation remainedsatisfactory. Only in the event of the extremely unlikely simultaneouslyoccurrence of a large spurious quadrature signal and of a considerableamplifier phase drift would there occur an error in flow indication, anda large reading on the meter 173 would make this situation immediatelyobvious.

FIGURE 5 shows an electrode assembly which is particularly adapted foruse when the liquid to be measured may contain oily or greasy materialswhich may provide insulated coating on the electrodes, such liquidsbeing typified by sewage or the like. In this figure the wall of theconduit 2 is indicated at 210, and in this case the tube is indicated aswholly formed of insulating material. If a metallic tube such as a tube2 previously described is used, it will be understood that the electrodeassembly will be inserted into an insulating bushing which, as alreadydescribed, may form a part of an insulating lining. Threaded into thetube is a connector 212 which is internally threaded to receive anextension of tube 214, there being provided a packing between them inthe form of an O-ring 216 which, together with an O-ring 213 between aflange on the connector 212 and the tube wall forms part of the packingto prevent escape of the flowing liquid. The extension of tube 214 has abore 218 which communicates with the interior of the tube 210. Desirablythe end of the extension of 214 projects slightly within the inner wallof the tube 210. The outer portion of the tube 214 has an enlarged bore220. A brush 222 formed by winding twisted stainless steel wire in adouble spiral about a stem 224 is located within the bore 218. The stem224 is mounted in a head 226 which is packed for sliding movement in thebore 220 by an O-ring 228 and is provided with an extended portion 227secured to an insulating operating handle 230 whereby the entire brushassembly may be moved axially of the bore 218. A snap ring 229 preventsthe head 226 from moving out of the tube 214. A metallic shieldinghousing 232 surrounds the assembly and is held in position by insulatingcollars 234 and 236 associated with the O-rings 238 and 240 which servemerely for frictional mounting means, taking no part in the aspect ofpacking against liquid leakage. An insulated electrode lead is indicatedat 242.

Electrical-1y the electrode assembly just described func tions the sameas the electrodes 12 previously referred to. The advantage of thearrangement is that by back and forth movements of the brush 222 theportion of the bore 218 communicating with the interior of the tube 210may be kept clean and free of deposits. The brush, of course, may takevarious forms, either being provided by twisted wire or with wirebristles, or the like, the arrangement being such as to have a scrapingor abrading action on the bore 218. The electrode is in efiectconstituted by the left hand portion of the bore. The parts are metallicexcept for those specifically described as of insulating material, andconsequently electrical continuity between the active electrode surfaceand the wire 242 is provided. The wire 242 may be provided with agrounded sheath connected to the shield cylinder 232 which may be thusor otherwise grounded to the effective electrical ground of theapparatus.

What is claimed is:

1. A fiowmeter comprising a conduit for flowing liquid, electromagneticmeans providing a magnetic field transverse to said conduit, meanssupplying current to said electromagnetic means, electrodes exposed toliquid flowing through said conduit and located on a line extendingtransversely through said field to pick up signals generated by the flowof liquid through said field, and means for measuring said signals, eachof said electrodes comprising a metallic tube open to the liquid saidconduit, and means projectable through said tube to clean its bore, saidelectrodes being insulated from each other.

'2. A flowmeter comprising a conduit for flowing liquid, electromagneticmeans providing a magnetic field transverse to said conduit, meanssupplying current to said electromagnetic means, electrodes exposed toliquid flowing through said conduit and located on a line extendingtransversely through said field to pick up signals generated by the flowof liquid through said field, and

, means for measuring said signals, each of said electrodes comprising ametallic tnbe open to the liquid in said conduit, and conductive meansprojecta-ble through said tube to clean its bore, said electrodes beinginsulated from each other.

3. A flowmeter comprising a conduit for flowing liquid, electromagneticmeans providing 'a magnetic field transverse to said conduit, meanssupplying current to said electromagnetic means, electrodes exposed toliquid flowing through said conduit and located on a line extendingtransversely through said field to pick up signals gem erated by theflow of liquid through said field, and means for measuring said signals,each of said electrodes comprising a metallic tube open to the liquid insaid conduit, means projectable through said tube to clean its bore andmeans packing said means withinthe tube to prevent escape of liquid fromsaid conduit, said electrodes being insulated from each other.

4. A flowmeter comprising a conduit for flowing liquid, electromagneticmeans providing a magnetic field transverse to said conduit, meanssupplying current to said electromagnetic means, electrodes exposed toliquid flowing through said conduit and located in a line extendingtransevrsely through said field to pick up signals generated by the flowof liquid through said field, means for measuring said signals, each ofsaid electrodes comprising a metallic tube open to the liquid in saidconduit, and conductive means projectable through said tube to clean itsbore, said electrodes being insulated from each other, and a conductiveshield surrounding each electrode tube and its cleaning means.

5. A flowmeter comprising a conduit for flowing liquid, electromagneticmeans providing a magnetic field transverse to said conduit, meanssupplying current to said electromagnetic means, electrodes exposed toliquid flowing through said conduit and located on a line extendingtransversely through said field to pick up signals generated by the flowof liquid through said field, means for measuring said signals, each ofsaid electrodes comprising a metallic tube open to the liquid in saidconduit, means projectable through said tube to clean its bore and meanspacking said means within the tube to prevent escape of liquid from saidconduit, said electrodes being insulated from each other, and aconductive shield surrounding each electrode tube and its cleaningmeans.

References Cited in the file of this patent UNITED STATES PATENTS

